An electric power steering apparatus (EPS) which provides a steering system of a vehicle with a steering assist torque (an assist torque) by a rotational torque of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears by using a driving force of the motor which is controlled by electric power supplied from an electric power supplying section (an inverter). In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feed-back control of a motor current. The feed-back control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a handle 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a pinion-and-rack mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the steering shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the handle 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting the steering torque of the handle 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command (a steering assist command) on the basis of the steering torque Th detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for the EPS by means of a voltage control command value Vref obtained by performing a compensation or the like to the current command value.
As well, the steering angle θ is detected from the steering angle sensor 14, and it is possible to obtain the steering angle from a rotational sensor such as a resolver which is connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 100, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
The control unit 30 mainly comprises a CPU (Central Processing Unit) (including an MPU (Micro Processor Unit) and an MCU (Micro Controller Unit)), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
The control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31 which calculates the current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1, based on the steering torque Th and the vehicle speed Vs with reference to an assist map or the like, which is a control target value of a current supplied to the motor 20. The calculated current command value Iref1 is inputted into a current limiting section 33 via an adding section 32A, and the current command value Irefm whose maximum current is limited is inputted into a subtracting section 32B. A deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im which is fed-back is calculated at the subtracting section 32B, and the deviation I is inputted into a proportional-integral-control section (PI-control section) 34 for improving a current characteristic of the steering operation. The voltage control command value Vref that the characteristic is improved at the PI-control section 34, is inputted into a PWM-control section 35, and the motor 20 is PWM-driven through an inverter 36. The current value Im of the motor 20 is detected by a motor current detector 37 and is fed-back to the subtracting section 32B. The inverter 36 is constituted by a bridge circuit of field-effect transistors (FETs) as a semiconductor switching device.
The rotational sensor 21 such as the resolver is connected to the motor 20 and a motor rotational angle θ is outputted. Further, a motor velocity ω is calculated at a motor velocity calculating section 22.
Further, a compensation signal CM from a compensation signal generating section 38 is added at the adding section 32A. A characteristic compensation of the steering system is performed by adding the compensation signal CM, and a convergence, an inertia characteristic, and the like are improved. The compensation signal generating section 38 adds a self-aligning torque (SAT) 38-1 to an inertia 38-2 at an adding section 38-4. The adding result is further added with a convergence 38-3 at an adding section 38-5. The adding result at the adding section 38-5 is treated as the compensation signal CM.
In a case that the motor 20 is a three-phase brushless motor, details of the PWM-control section 35 and the inverter 36 have a configuration as shown in FIG. 3, and the PWM-control section 35 comprises a duty calculating section 35A that calculates the PWM-duty values D1 to D6 which are used in a three-phase PWM-control by using the voltage control command value Vref in accordance with a predetermined equation, and a gate driving section 35B that drives gates of the FETs as the driving device by means of the PWM-duty values D1 to D6 and turns-ON or turns-OFF the gates of the FETs for compensating a dead time. The inverter 36 is constituted by the three-phase bridge of the FETs (FET 1 to FET 6) as the semiconductor switching device, and the motor 20 is driven by turning-ON or turning-OFF the gates of the FETs with the PWM-duty values D1 to D6. Motor relays 23 for supplying (ON) the electric power or blocking off (OFF) the electric power are connected to respective phases in the electric power source lines between the inverter 36 and the motor 20.
When the motor current is not accurately controlled in the electric power steering apparatus which uses such a brushless motor, the generated torque by the motor cannot be accurately obtained. Thus, the sensor which precisely detects the motor angle is required. The resolver, a Hall sensor, a magneto resistive sensor (an MR sensor) and the like are used as the angle sensor, and any of the above sensors have a function that detects the motor angle with a required accuracy.
FIG. 4 shows an overall configuration of an angle detecting system, and a torsion bar 2A is provided with the steering shaft (the column shaft) 2 which is connected to the handle 1. A handle-side angle sensor 14H is disposed at the handle 1 side of the torsion bar 2A and a pinion-side angle sensor 14P is disposed at the pinion side of the torsion bar 2A. The rotational sensor 21 such as the resolver is attached to the motor 20 which is provided with the steering shaft 2 through the reduction gears 3. A pinion-side angle θp detected at the pinion-side angle sensor 14P, a motor angle θm detected from the rotational sensor 21 and a motor current Im detected at a current detecting circuit are inputted into the control unit 30.
However, the electric power steering apparatus using the brushless motor is used to the vehicles which have a relatively large rack thrust force due to reasons of a high motor efficiency and so on. Therefore, since it is impossible to control the motor current when the motor angle sensor is failed, there is a problem that the assist function is failed and the driver feels a quite different steering feeling relative to the steering feeling in a normal operation. Further, since the failure of the assist function means that the force assisted by the motor till then is suddenly lost, the driver receives a force, which is corresponding to the assist force of the motor, as a sudden reaction force from the handle. This causes the driver to receive an impact from the handle. In a case that the impact is in a range to be controlled by the driver, the problem is small. Reversely, in a case that the impact is out of the range to be controlled by the driver, since not only an influence to the driver, but also the influence to the vehicle behavior can be occurred, this can cause the problem of the safety.
In this connection, the electric power steering apparatus adopting the brushless motor is strongly required to be equipped with an alternative means which can perform the angle detection even when the angle sensor of the motor is failed.